(1) Field of the Invention
The invention relates to a screen, and especially relates to a projection screen.
(2) Description of the Prior Art
Projection screen is a kind of optics screen used to diffuse projection light from a projector. When direction of the projection light and the luminous flux are fixed, the ratio between luminance of the light reflected by the projection screen and the ideal luminance acceptable by eyes is defined as a luminance coefficient in the direction of the light reflected by the projection screen. The maximum luminance coefficient is defined as the gain of the projection screen, representing the reflection capacity of the projection screen. Generally, to enhance image viewing angle, contrast, and gain, the projection screen is not only required to diffuse the projection light of the projector, but also required to eliminate interference of external light to avoid influence on the image contrast.
FIG. 1 is a schematic view of a conventional reflective front projection screen 100. Referring to FIG. 1, an opaque substrate 110 is disposed on the bottom layer of the reflective front projection screen 100. A transparent glass ball structure 120, and two transparent material layers 130 and 140 with different but similar indexes of refraction are disposed on the opaque substrate 110 orderly. A lens structure 150 composed by a plurality of lenses arranged continuously is disposed on a surface of the transparent material layer 140. One surface of the lens structure 150 is covered by a black shield layer 160.
After entering the projection screen 100 from another surface of the lens structure 150, a projection light R first passes the two transparent material layers 130 and 140 with different indexes of refraction, and then enters the glass ball structure 120. For the bottom layer of the projection screen 100 is an opaque base 110, the projection light R may be reflected or absorbed. The reflected beam may pass through the glass ball structure 120, and exit out of the projection screen 100 along an emergent light path R2 or R3. The glass ball structure 120 may diffuse the emergent light to increase the image viewing angle. Moreover, the black shield layer 160 on the lens structure 150 may absorb external miscellaneous light O to avoid the image contrast reducing by the influence of the external miscellaneous light O.
FIG. 2 is a schematic view of another conventional reflective front projection screen 200. Referring to FIG. 2, an opaque base 210 is disposed on bottom layer of the reflective front projection screen 200. A transparent glass ball structure 220, and two transparent material layers 230 and 240 with different but similar indexes of refraction are disposed on the opaque substrate 210 orderly. A surface 250 of the transparent material layer 240 is fabricated into a plane structure.
After entering the projection screen 200 from the surface 250 of the transparent material layer 240, a projection light L first passes the two transparent material layers 230 and 240 with different indexes of refraction, and then enters the glass ball structure 220. For the bottom layer of the projection screen 200 is an opaque base 210, the light beam L may be reflected or absorbed. The reflected beam passes through the glass ball structure 220, and exits out of the projection screen 200 along an emergent light path L2 or L3. The glass ball structure 220 may diffuse the emergent light to increase image viewing angle.
However, the reflective front projection screens 100 and 200 have following questions respectively:
Firstly, the lens structure 150 of the reflective front projection screen 100 in FIG. 1 uses the black shield layer 160 to absorb the external miscellaneous light O, but a part of the emergent light, such as the emergent light along the emergent light path R2, may be absorbed by the black shield layer 160, thus light emergent efficiency and the gain of the projection screen may be decreased.
Secondly, though the fabrication of the reflective front projection screen 200 in FIG. 2 is quite simple, it may not prevent the miscellaneous light O. For example, the external miscellaneous light O may enter the projection screen 200 from the emergent light path L2 or L3 to decrease the image contrast.
Thus, how to increase the image contrast and the gain of the projection screen is an urgent problem in the technology field.